Bone marrow

Bone marrow is ubiquitous throughout the skeleton, primarily composed of hematopoietic cells and fat cells between bony trabeculae and fibrous retinacula. It performs numerous physiological functions and dynamically changes during normal aging and in response to stressors and pathology. Although bone marrow can be seen or inferred on many imaging modalities, MRI is best able to visualize its constituents directly.

Gross anatomy

Bone marrow contains:

• hematopoietic cells

• supporting stroma

• reticulum (phagocytes and undifferentiated progenitor cells)

• scattered fat cells

• rich vascular supply

Although all normal bone marrow contains the same constituents, the proportion of hematopoietic cells and fat varies dramatically depending on age and region. This allows normal bone marrow to be divided into red and yellow marrow based on how much fat it contains. 

Red marrow

Not only is red marrow is most abundant in infancy, occupying the entire skeleton (see below), but it is also contains composed primarily of haemopoeitc cells. At birth red marrow comprises of almost 100% hematopoietic cells. Over time this component reduces with increased contribution of fat cells such that by young adulthood the hematopoietic component has reduced to 60% ¹⁰.

By weight, the red marrow of adults is composed of 40% water, 40% fat and 20% protein ¹⁰.

Yellow marrow

Conversely yellow marrow, although having the same constituents as red, is dominated by fat (15% water, 80% fat, 5% protein) with little vascularity ¹⁰.

Normal marrow conversion

During infancy, red marrow occupies the entire ossified skeleton except for epiphyses and apophyses. Gradually, red marrow "retreats" centrally, such that by 25 years of age, it is essentially confined to the axial skeleton (pelvis, spine, shoulder girdle, skull). The conversion of red to yellow marrow progresses from distal to proximal of the extremities, so first hands and feet, then forearms/lower legs, then humeri/femora, then pelvis/spine.

Within the long bones, the epiphysis is the first to undergo conversion followed by the diaphysis before extending to the metadiaphysis ^5,6.

Although the distribution varies with age and from one individual to another, it should be symmetric.

Heterogeneous bone marrow signal is common and can be challenging to distinguish from pathology. Islands of red marrow may also be seen anywhere in the skeleton, typically in a subcortical distribution, often with central yellow marrow giving it a bull' s-eye appearance on axial imaging. Additionally, red marrow is found in subchondral crescents; typical locations include the proximal humerus and femur ². Similarly, focal fatty deposits in bone marrow may be seen essentially anywhere within the skeleton.

Yellow marrow can also be seen focally within vertebral bodies. The pattern of normal red and yellow bone marrow distribution in the spine is variable ⁹:

• yellow marrow visible around the basivertebral veins (particularly common in younger patients, predictably seen following regression of pediatric widespread red marrow)

• band-like and triangular-like areas of yellow marrow in the vertebral body corners and abutting the endplates (adjacent to degenerative disc disease and Schmörl nodes)

• speckled pattern (punctate foci of red and yellow bone marrow)

• ​​larger areas of yellow marrow and poorly circumscribed areas of red marrow

Radiographic features

MRI 

• T1

  • red marrow: hypointense to subcutaneous fat, but hyperintense to muscle and disc (due to scattered fat cells)

  • yellow marrow: hyperintense (follows the signal of subcutaneous fat)

• T2

  • red marrow: slightly hyperintense to muscle, usually its signal intensity is slightly lower than that of yellow marrow, but sometimes it can be difficult to distinguish the two 

  • yellow marrow: hyperintense to muscle and iso- to slightly hypointense to subcutaneous fat

• STIR

  • red marrow: remains hyperintense

  • yellow marrow: is saturated out (hypointense)

• T1 C+ (Gd)

  • normal bone marrow in adults does not enhance visibly, whereas there may be a significant contrast enhancement in normal marrow of a neonate or a small child

  • enhancement in adults occurs only in pathological marrow, which can be accentuated on post-contrast images with fat saturation

Related pathology

Bone marrow can demonstrate changes as a result of a very wide variety of pathologies, either due to direct involvement or as a response to disease elsewhere.

• bone marrow edema

• bone marrow conversion

• bone metastases

• hemangiomas of bone

• myeloproliferative neoplasms (see WHO classification of haematolymphoid tumors)

• myelodysplastic syndromes

• serous atrophy of bone marrow